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Screws made to measure

Centrifuges separate substances of different densities, and, depending on the material, are constructed in different ways. Of a total of 9,000 machines manufactured by Flottweg SE, more than 5,000 differ in significant details. This places high demands on the manufacturing technology, in particular the centrifuge screws.

Individual solutions for industrial centrifuges

From sewage treatment to separation tasks in mining to sorting plastics in recycling, there are many applications for separation technologies and just as many different machines. Flottweg SE, a manufacturer of industrial centrifuges headquartered in the Upper Bavarian town of Vilsbiburg, customizes its machines to the requirements of the various separation tasks. These centrifuges function more smoothly and are more powerful than ready-made solutions.

The right screw for every task: the screw inside the centrifuge

During operation, the “screw” inside the centrifuge turns inside a cylindrical-conical drum, which also rotates. Almost every screw manufactured by Flottweg is unique. The right pitch, the optimal inlet location, the appropriate cone, and the correct gap between the screw and the rotor drum are all decisive factors in the system’s performance. The screws are welded constructions that require numerous machining ­operations on the turning machine. They can weigh up to 1.5 tons and be several meters long.

“For the more than 9,000 centrifuges that we have produced, we have designed and manufactured around 3,500 different screws,” explains Josef Geltinger, production foreman at Flottweg. “This is because we often produce our systems for niche ­applications and make them out of many different high-alloyed steels, and also because we emphasize providing our customers with tailor-made solutions for all applications.”

The right program for every screw

The turning machine operators work with around 2,000 CNC programs when producing the screws, all of which they have created themselves, and most of which have been further adapted in detail. The programming takes place directly on the machine with the drawing in hand, for example, on a Seiger SLZ 1200E cycle turning machine with Sinumerik 840D sl. The cycle or flatbed turning machines are used primarily in individual part and small-batch production. This type of machine places special ­demands on the CNC. The control system must automate the machining process, as in a normal CNC universal turning machine, but it must also guarantee that the operation and programming of the individual-part production is flexible and easy to understand. The shopfloor-oriented ShopTurn work-step programming is ideal for this approach, as all machining steps are stored as cycles. With the Manual Machine function, all the familiar standard cycles from ShopTurn, such as stock removal, grooving, undercut, thread and so on, can be parameterized and started in jog mode, without having to write a program. But the sky is the limit. If a complete NC program needs to be run automatically, the workpiece can be completely programmed in ShopTurn as usual.

The high level of accuracy and rigidity of the turning spindle and the torsion-resistant box bed create the required stability for this machine over the entire length of 5,000 mm and a swing diameter of up to 1,450 mm. Micrometer-level accuracy is required when producing the screws. For example, the outside contour must be exact along the entire length after the smoothing process, as the resulting gap ­between the screw and the rotor drum significantly affects the separation results.

High material requirements

The material requirements are very high, as centrifuges need to withstand extreme loads. When both the screw and the drum casing (rotor) are operating, the systems reach up to 5,000 rpm. The acceleration forces on the material to be separated can then increase to up to 10,000 times the acceleration of gravity. If there is a mixture of sandlike sediments, this is very similar to constant machining with sandpaper at extremely high pressure.

Wear materials for the screws are tungsten carbide, carbide metal and stellite. The screw body itself is made of hard chrome-nickel steel. “We know the machining parameters very well,” says Geltinger. “There can’t be any waste with these expensive materials.” The easy parameterization of the ShopTurn cycles provides a great deal of security here. The screen masks are structured simply and clearly, and the cutting parameters that have already been entered remain in the cycle until the operator changes them. This is why the Flottweg factory works with tried-and-tested machine technology – and in the future the company will use exclusively SINUMERIK controlled turning machines. “The staff members are ­experienced with ShopTurn, and we are also satis­fied with the service offered to us by the regional Siemens branch,” stresses Geltinger.

The Manual Machine function

For cycle-controlled turning machines, the Manual Machine function is available on SINUMERIK. This additional operating option allows for workpiece machining in manual operation – without having to write a parts program. The individual cycles are integrated in the bottom bar, are selectable and support the handwheels.